Nickel is an essential trace element which is found in enzymes involved in the metabolism of hydrogen, carbon monoxide and urea as well as the synthesis of acetyl coenzyme A and methane. Exposure to excess nickel has been linked to cancer. Soluble nickel compounds can cross the placenta and act on the fetus. Nickel deficiency in numerous laboratory animals causes simultaneous iron, calcium, and zinc deficiency. Nickel is present in human blood serum and three nickel binding fractions have been identified. NIckel is thought to be the active catalytic sites of certain hydrogenases and carbon monoxide dehydrogenases. Present data indicates that these enzymes each possess unique nickel sites that are rich in sulfur ligating atoms, redox active and intimately involved in catalysis. The research proposed here is directed toward gaining an understanding of the nickel sites of these enzymes. The objectives of this project are to synthesize and to structurally, chemically, and spectroscopically characterize nickel complexes which are models of the nickel sites of these enzymes. Special attention will be given to nickel(III) compounds in sulfur rich environments and to nickel compounds with rigid or semi-rigid, sterically constraining ligands providing dominant sulfur coordination. The complexes and their reactions with substrates will be studied by electron paramagnetic resonance spectroscopy (EPR), electrochemical methods, and x- ray crystallography.